Plenarvorträge - DPG-Tagungen

Dynamik und Statistische Physik Donnerstag
presence of shear flow. Generalizing the approach of [2], ferrofluid droplets
are modeled as deformable and orientable ellipsoids of revolution.
The combined effects of a magnetic field and a shear flow on the orientation
and deformation of the droplets are studied. It is found that the
hydrodynamic drag leads to a flow alignment of the droplet orientation.
Predictions for the alignment angle as a function of the applied field are
presented. Some comparisons to experimental data [3] in the absence of
shear are made.
[1] E. Blums, A. Cebers, M.M. Maiorov, Magnetic Fluids, deGruyter,
Berlin 1997.
[2] P. Ilg and M. Kröger, Phys. Rev. E 66, 021501-1 (2002)
[3] A. Lebedev, A. Engel, K.I. Morozov and H. Bauke, New Journal of
Physics 5, 57.1-57.20 (2003)
DY 46.81 Do 16:00 Poster D
The corkscrew instability in a nematic liquid crystal —
•Alberto de Lozar, Wolfgang Schoepf, Lorenz Kramer, and
Ingo Rehberg — Universitaet Bayreuth
A liquid crystal with slightly positive dielectric anisotropy is investigated
in the planar configuration. This system allows for competition
between electroconvection and the homogeneous Frèedericksz transition,
leading to a rather complicated bifurcation scenario. We report measurements
of a novel instability leading to the ”corkscrew” pattern. This state
is closely connected to the Frèedericksz state as it manifests itself as a regular
modulation along a Frèedericksz domain wall, although its frequency
dependence indicates that electroconvection must play a crucial role. It
can be understood in terms of a pitchfork bifurcation from a straight
domain wall. Quantitative characterization is performed in terms of amplitude,
wavelenght and relaxation time. Its wavelenght is of the order of
the probe thickness, while its ondulation amplitude is an order of magnitude
smaller. The relaxation time is comparable to the one obtained for
electroconvection.
DY 46.82 Do 16:00 Poster D
Magneto-rheological study in inverse magnetic fluids using
polystyrene particles — •Saldivar-Guerrero Ruben 1 , Richte
Reinhard 1 , Rehberg Ingo 1 , and Rodriguez-Fernandez Oliverio
2 — 1 Universitaet Bayreuth, Experimentalphysik V — 2 Centro de
Investigacion en Quimica Aplicada
Ferrofluids are colloidal suspensions of magnetic nanoparticles, which
change their behaviour under control of an external magnetic field. But
when for example, they are mixed with polystyrene particles, they present
another interesting properties. The magneto-rheological fluid (MRF) obtained
in this way, can show a solid-fluid transition by an application
of a moderate magnetic field similar to electro-rehological fluids (ERF)
under a electric field. It excels ERF’s because only moderates currents
instead of high voltages are necessary. We study the rheological influence
of the diameter of the immersed polymer particle in MRF and the relation
between the coupling parameter l and the yield point ty for different
magnetic fields applied. The results show that the rheological properties
are increased when the particle diameter increases in a constant magnetic
field. Also we investigate structure formation in the fluid by video
microscopy when a magnetic field is applied.
DY 46.83 Do 16:00 Poster D
Smectic and crystalline order in colloidal monolayers on periodic
one-dimensional substrate potentials — •Jörg Baumgartl
1 , Matthias Brunner 2 , and Clemens Bechinger 1 — 1 2.
Physikalisches Institut, Universität Stuttgart, Pfaffenwaldring 57, 70550
Stuttgart — 2 Fachbereich Physik, Universität Konstanz, 78457 Konstanz
There is growing interest in the phase behavior of two-dimensional
(2D) colloids in the presence of one-dimensional (1D) periodic substrate
potentials. The phase behavior of such systems is determined by two factors:
(i) the relative orientation between the 2D crystal and the periodic
potential troughs which selects a set of Bragg planes running parallel to
the troughs and (ii) the commensurability ratio p = a ′ /d of the spacing
a ′ between these Bragg planes to the period d of the periodic potential.
So far, only the phase behavior for p = 1 was investigated and the following
phases were observed for increasing (non zero) potential strength:
modulated liquid and light-induced crystal. For other commensurability
ratios additional, more complex phases are predicted.
We investigated experimentally the phase behavior for the case p = 2
with the 1D periodic potential being created by two interfering laser
beams. We observed for the first time the theoretically predicted intermediate
phase between modulated liquid and light-induced crystal which
is termed locked smectic. By analyzing the structure factor and the pair
correlation function, this phase is characterized by liquid-like behavior
along the potential troughs. In contrast to the modulated liquid the ordering
perpendicular to the troughs is similar to the crystalline state,
with particles occupying only each second trough.
DY 46.84 Do 16:00 Poster D
Commensurability effects of colloidal monolayers on hexagonally
patterned substrates — •Stefan Bleil 1 , Matthias Brunner
2 , and Clemens Bechinger 1 — 1 2. Physikalisches Institut, Universität
Stuttgart, Pfaffenwaldring 57, 70550 Stuttgart — 2 Fachbereich
Physik, Universität Konstanz, 78457 Konstanz
Colloidal suspensions are excellent model systems for investigations
of phase transitions in two-dimensional (2D) systems. While most of
the previous work concentrated on homogeneous substrates, much less
is known about the phase behavior on patterned substrates although
the latter closely resembles the behavior of atomic adsorbates on crystalline
surfaces. In our experiments we mimic 2D patterned substrates
for colloidal particles by the interference pattern of several laser beams
which lead to triangular substrate potentials. We present results where
the number density of charge stabilized colloidal particles was systematically
varied. This allowed us to study the effect of the filling fraction
η (i.e. the ratio between the number of colloidal particles and the number
of substrate wells) on the observed structure of the colloidal system.
Depending on the filling fraction we observe commensurate and incommensurate
structures. We also compare our results with recent numerical
simulations.
DY 46.85 Do 16:00 Poster D
Phasetransitions in Quantum fluids: PIMC - Studies — •Guido
Günther and Peter Nielaba — Lehrstuhl für Theoretische Physik,
Fachbereich Physik, Universität Konstanz, D-78457 Konstanz
Exploiting path integral Monte Carlo methods we studied the behavior
of the fluid phase of quantum mechanical hard sphere- and disk- systems.
We focused on the superfluid phase transition of Bose systems
analyzing, among other quantities, the superfluid fraction which in
periodic boundary conditions is related to the winding number of the
Trotter-chains [0].
[0] E.L. Pollock, D.M. Ceperley; Phys. Rev. B 36, 8343 (1987)
DY 46.86 Do 16:00 Poster D
Microcanonical entropy of classical spin systems with a continuous
symmetry — •Andreas Richter, Michel Pleimling,
and Alfred Hüller — Institut für Theoretische Physik I, Universität
Erlangen-Nürnberg, D-91058 Erlangen
Critical phenomena of classical spin systems may be studied in many
ways. In our approach we determine and analyse the microcanonical entropy
S which is related to the density of states by S = kB ln Ω, where kB
is the Boltzmann constant. Due to the huge amount of possible configurations
it is in general impossible to calculate Ω exactly, even for systems
of moderate size. The density of states can, however, be computed approximately
from a numerical method using transition variables. Discrete
classical systems, like the Ising or Potts model, were already studied with
this method in the past [1,2]. We now extend this method to systems with
continuous spins and apply it to the three-dimensional XY model. In addition,
we discuss the determination of critical quantities directly from
the numerically determined microcanonical entropy.
[1] A. Hüller, M. Pleimling, Int. J. Mod. Phys. C 13, 947 (2002)
[2] H. Behringer, J. Phys. A: Math. Gen 36, 8739 (2003)
DY 46.87 Do 16:00 Poster D
A Topological Rule-of-Thumb for Percolation Thresholds —
•Richard Neher and Herbert Wagner — Sektion Physik, Universität
München
Randomly assembled structures exhibit a density driven percolation
transition, beyond which an unbounded component is formed. Since properties
of those structures change dramatically at the percolation threshold,
its accurate knowlegde is important and various empirical formulas,
predicting percolation thresholds, have been suggested. The mean Euler
characteristic is a topological measure for such random sets and it has
been suggested to use its zero crossing to estimate the threshold. In contrast
to other formulas, this estimation does not rely on a fit to known
thresholds.
We further investigated this matter and found, that for a great variety